This invention relates generally to xerographic copying apparatus, and more particularly, it relates to fixing of particulate thermoplastic material arranged in image configuration by passing the substrate carrying the images between a pair of unheated pressure engaged roll members forming part of a three roll pressure fuser.
In the process of xerography, a light image of an original to be copied is typically recorded in the form of a latent electrostatic image upon a photosensitive member with subsequent rendering of the latent image visible by the application of particulate thermoplastic material, commonly referred to as toner. The visual toner image can be either fixed directly upon the photosensitive member or transferred from the member to another support, such as a sheet of plain paper, with subsequent affixing of the image thereto, the most common method of affixing comprising the simultaneous application of heat and pressure.
In order to affix or fuse electroscopic toner material onto a support member by the simultaneous application of heat and pressure it is necessary to elevate the temperature of the toner and simultaneously apply pressure sufficient to cause the constituents of the toner to become tacky and coalesce. This action causes the toner to flow to some extent into the fibers or pores of support member or otherwise upon the surface thereof. Thereafter, as the toner material cools, solidification thereof occurs causing the toner material to be bonded firmly to the support member. In both the xerographic as well as the electrographic recording arts, the use of the above-described fuser for fixing toner images onto a support member is old and well known.
It is equally well known that heat and pressure fusing of toner images has various drawbacks. For example, a large quantity of electrical power is required to raise the surface temperature of the fuser roll to a suitable temperature. Approximately 70% to 80% of the total power consumed by the entire copying machine is needed for this type of fuser. Another disadvantage resides in the fact that heat and pressure fusers require a relatively long warm up period. Moreover, the fuser roll is heated during stand-by which results in a waste of thermal energy. As will be appreciated, the other components of the machine may be adversely affected by the heat dissipated into the machine environment and in the case of very high speed reproducing apparatus cooling systems are required.
In view of the disadvantages noted above with respect to heat and pressure fusing, other types of fusers have been pursued. One of the more actively pursued areas in this respect is cold pressure fusers. First attempts to fuse toner images by pressure along (i.e. without the aid of heat) were accomplished by a pair of rolls to which the required pressure was applied. Due to the high pressures involved, problems such as roll bending were encountered. Such a problem has been solved by skewing the two rolls relative to each other or by crowning or by centrally supporting one or both of the rolls.
Further developments in the area of cold pressure fusing resulted in three roll systems which, in general, are less expensive than the two roll systems because the rolls are smaller. An example of a two roll system is described in U.S. Pat. No. 3,854,975 while a three roll system is described in U.S. Pat. Nos. 4,192,229 and 4,259,920.
Not only are known two roll systems more expensive than the three roll fusers they also are less subject to paper wrinkle. This is because in a two-roll fuser the rolls are usually skewed in order to preclude adverse bending of the rolls when the load is applied. Skewed rolls tend to induce corner curl in the copy sheets and laterally translate the paper in the nip, the later of which can be a problem for short paper path configurations. With a three-roll fuser the axes of the rolls forming the nip through which the copy sheets pass can be parallel while the third roll which applies the load can be skewed. Such an arrangement is disclosed in U.S. Pat. No. 4,259,920. As illustrated therein the skewed backup or loading roll is substantially shorter than the nonskewed rolls. Alternatively the rolls forming the nip have been skewed but this arrangement causes proper wrinkle.
Three-roll systems where the nip is formed by two parallel rolls also have an advantage over two-roll systems in that once they are set up using one size paper they can accommodate various size papers. In a simply supported two-roll skewed system which is set up using one size paper, a change in paper size results in a change in load distribution which results in a change in deflection of each roll in the opposite direction.
I have found that the three-roll fuser with the third roll skewed is prone to edge wear which results in poor copies. Edge wear is the wear on the pressure roll where it is contacted by the edges of the skewed roller. Edge wear is caused by the loading forces exerted at the edges of the skewed roll where they contact the pressure roll. Thus, do to the forces exerted by the edges of the skewed loading roll on the pressure roller it becomes grooved. Since the grooves are well within the paper path these grooves adversely affect the final copy.
Another problem that I have found with roll pressure fusers is that paper jams are difficult to clear because these fusers do not provide for unloading of the rolls to facilitate jam clearance. In prior art cold pressure fusers, the loading of the rolls is accomplished at each end of the roll to which loading is required. In a two-roll arrangement, each end of the pressure roll is provided with a separate loading device such as a spring and a set screw. The set screw is adjustable to set the desired load which requires some means for measuring the load exerted in the nip. Once the two set screws have been adjusted to the required load they cannot be disturbed without redoing the entire procedure. Accordingly, a paper jam has to be cleared with the load on the rolls.